Huge agribusiness combines in the San Joaquin Valley, the richest and most densely cultivated agricultural region in the nation, are fast adopting "organic" methods of pest control and drastically reducing their use of chemical sprays because they've discovered it saves them a lot of money.

The growers range from the 45,000-acre J. G. Boswell Corp., the largest irrigated farming operation in the United States, and Prudential Insurance Co.'s huge Dudley Ridge Ranch, to small family-owned plots.

The farmers are turning to a loosely defined concept known as "integrated pest management," or IPM. IPM techniques, developed by university-trained advisers, combine traditional concepts of crops rotation with sophisticated bug traps and computer analysis of insect life cycles that show best how to interrupt them. IPM uses chemical poisons only as a last resort.

While "organic" or chemical-free farming is often promoted by city-bred environmentalists, the most enthusiastic proponents of IPM scoff at the suggestion that they are being moved by a change of social philosophy. "We're motivated by economics, pure and simple," explained Mike Shannon, a third-generation crop farmer whose family-owned S-K Ranch owns or has interest in some 30,000 acres.

"None of us in farming wants to spend a dime on anything - whether it's machinery, labor, or spray," he said.

Indeed, Shannon maintains that it is the large grower who is best able to put so-called "natural" methods, such as corp rotation, into practice. "It doesn't hurt us, it helps. But small farmers can't let their land lie fallow."

According to the U.S. Environmental Protection Agency, 80 percent of the 1 billion pounds of pesticides used by American farmers each year are petroleum-based and thus have developed natural resistance to many pesticides, and health hazards to humans have led to government bans on others.

Shannon said the S K Ranch has been able to reduce its pesticides use by two-thirds while "getting the same production we got in our peak periods of 30 years ago."

And Shannon is not alone. Although more land in the area is coming under cultivation, making use rates difficult to compute, an estimated 30 per cent of the valley's farmers now use IPM methods.

The S-K ranch owns a pesticide supply company and a crop-dusting service, but Shannon says he'd rather not spray. "It costs money," he says. "We have the planes, but I'd rather not touch them."

Shannon says he has little regard for statements by Secretary of Agriculture Bob Bergland, who, he says, suggested that farmers are resisting alternatives to massive pesticide use.

"Bergland talks like we never heard of it [IPM.] We get the impression that they think all farmers are stupid, waiting for every chance to spray."

Shannon acknowledges he was first introduced to IPM by Richard Clebenger, a 36-year-old agronomist who grew up on a small farm nearby. Coincidentally, Clebenger, one of the few IPM advisers in this area, first began work as a pesticide salesman for a large chemical company.

Then, about a dozen years ago, Clebenger was hired to manage a small farm. There he got his chance to put to use some of the principles he had learned in his agronomy training. Despite good results, however, others werehard to convince.

Clebenger says there was great resistance at first to IPM techniques. "There were - and still are - a lot of farmers we call 'dusty,' guys who can't sleep right unless they given their fields a good spray."

In 1970 Shannon set aside a 200-acre cotton patch on which Clebenger could try his new techniques. "That was my big break," Clebenger recalled. "I proved it to him in dollars and cents."

Since then Clebenger has gone from working "out of my back pocket" to a $400,00 a year IPM consulting business, in which he uses a Beechcraft Bonanza to call on his more than two dozen clients.

Driving down a rutted rain-washed road, Clebenger and Shannon pointed out one application of IPM. On one side was a new planting of cotton.

By early spring, Clebenger will have a series of electric insect traps spotted among the alfalfa. Using deep purple light bulbs as lures, the traps, five-foot-high clylinders, will catch bugs for a daily count.

Keeping a tally of the numbers and types of bugs present, and matching that against computer analyses of pest movement and mortality patterns, Clebenger can determine the balance of parasites and their predators and what he is likely to have trouble with and when. In an extreme case, his data may tell him he needs to spray, but more often there is an organic remedy.

In the case of the cotton, he will leave a wide jump the road onto the ripe cotton buds.

"If we cut it all down the lygus bugs would jump to the cotton," says Clebenger. "We'd have to spray the cotton and kill everything, including the beneficial - the pirate bugs, the lady-bugs. After that, we'd have a boll worm buildup, a mite problem, or some other critters. It would never stop."

From the air, the pressure on agrciutlural production in this valley is clearly illustrated. Stretching from the granite foothills of the Sierra Nevadas, the land resembles a giant checker-board of green and rich brown. Giant oaks once dotted these plains, a sanctuary for birds which kept many insects in check. But the trees proved a barrier to machinery and they were rooted out, except for isolated stands kept by sentimental landowners as shelter from the hot summer sun.

In the old days., says Clebenger, good farmers did better than their neighbors because of their will to work and their knowledge of the crop. Today, chemicals are the great equalizer. "Pesticides can be the farmer's pacifier. Mine, too."

"I'm not an environmentalists, but the study of the environment is what this is all about. Look, we jerked out the weeds and the trees, along came the bugs which ate everything up," he said.

"I am what I started out to be, a field checker, someone who talks to farmers. We make a lot of educated guesses. We got insects that are alive and moving."